Hi, I was looking to buy a can (30lb) of R22 last August and remember it was about $150 average. I didn't buy it because the weather got cooler and I though I could wait till this year. To my surprise, everywhere I looked, the price is now around $300. Did the price go up or it's spring versus fall price?

R22 is no longer the current refrigerant and is being phased out. I believe it is now R-410A.
Just like when R22 replaced R12, the price of R12 sky-rocketed.
That and yes the seasonal aspect doesn't help either.

45% reduction? yeah, that explains the price jump. I was aware of the R22 being phased out but I though it would be a more gradual process.

Besides R410a which is not a direct replacement for r22, are there any alternatives to R22? There is something called R22a but I think it's a hydrocarbon, flammable variety. Is Dupont's ISCEON® MO99 any different? Can it be used to top off a R22 system without removing the existing R22?

45% reduction? yeah, that explains the price jump. I was aware of the R22 being phased out but I though it would be a more gradual process.

Besides R410a which is not a direct replacement for r22, are there any alternatives to R22? There is something called R22a but I think it's a hydrocarbon, flammable variety. Is Dupont's ISCEON® MO99 any different? Can it be used to top off a R22 system without removing the existing R22?

I wouldn't. Neither is cheap. If you are talking about a top-off I'd use the 22. Mixing would risk an evacuation and starting over which would be really costly.